The present invention relates to an injection molding machine in which a charging ram injects a settable material into the cavity of an associated mold. More particularly, the invention is concerned with a regenerative feed system which displaces the ram during the injection operation.
In prior art injection molding machines such as shown in U.S. Pat. Nos. 2,786,234 and 3,509,600, certain parameters of an injection molding process are controlled and modulated to carry out the molding process in a particular manner. For example, in a molding machine having a screw feeder or ram that is rotated within a barrel to develop a charge of settable material at one end of the barrel ahead of the ram, it is known to displace the ram with a controlled feed system and to inject the charge into a mold cavity at variable rates and pressures. During the fill portion of the ram stroke, ram speed or pressure may be modulated to control the rate at which the settable material flows into the mold. At the end of the fill portion of the stroke, it may be desirable to increase the ram packing pressure so that the material completely fills the mold cavity including any detailed impressions, and any pockets or voids caused by shrinkage of the material.
The pressure and speed modulation is controlled generally by means of the hydraulic system which operates a piston and cylinder assembly that displaces the ram. The pressure of the hydraulic fluid delivered to the piston and cylinder assembly may be regulated, and a change in such pressure provides a corresponding change in the pressure developed by the ram on the plasticized material in the mold.
It is also known to vary the feed rates of the ram by employing one or more pumps or fill valves in the hydraulic system that operates the ram. By increasing the flow through a multi-pump or valve system, a higher rate of ram displacement and correspondingly a shorter charging time is achieved. Naturally the use of multiple pumps or valves necessitates an associated control system to vary ram displacement. Changing the rate of ram displacement at an intermediate point in the charging stroke introduces an additional degree of complexity.
It is accordingly a general object of the present invention to provide a new and improved method and apparatus in an injection molding machine for controlling the speed or pressure at which the injection ram operates.